In portable radio devices, mobile communication devices in particular, the antenna is preferably located within the covers of the device for user convenience. An internal antenna of a small-sized device is usually a planar type antenna because in that case it is easiest to achieve an antenna with satisfactory electrical characteristics. A planar antenna includes a radiating plane and a ground plane parallel thereto. FIG. 1 shows an example of a known internal planar antenna with its feed arrangement. Depicted in the figure there is a circuit board 101 of a radio device, which circuit board has a conductive upper surface. This conductive surface serves as a ground plane 110 in the planar antenna. At one end of the circuit board there is the radiating plane 130 of the antenna, which radiating plane lies above the ground plane, supported by a dielectric frame 150. For matching of the antenna there is at the edge of the radiating plane, near a corner thereof, a short-circuit conductor 121, which connects the radiating plane to the ground plane, and the antenna feed conductor 122. These conductors are in this example of one and the same metal plate with the radiating plane, each at the same time providing a spring by the force of which they are pressed against the circuit board 101 when the antenna is in use. For the feed conductor 122 there is a lead-through, isolated from the ground, to an antenna port on the lower surface of the circuit board. Antenna matching is provided through proper location of the feed and short-circuit conductors, design of the radiating plane, and potential additional components. The antenna may be arranged to have multiple operating bands by dividing the radiating plane into two branches of different electrical lengths as viewed from the short-circuit point by a nonconductive slot.
A disadvantage of the structure shown in FIG. 1 is that when trying to achieve a very small device, the space required by the radiating plane within the device may be too big. In principle this disadvantage could be avoided if the radiating plane were fabricated as part of the cover of the device. This, however, would restrict the design of the radiating element and thus make it more difficult to achieve the electrical characteristics desired.
In the prior art, antenna structures are known which include a surface radiator fed by a primary radiator. FIG. 2 shows an example of such a structure. A surface radiator 230 is attached onto the inner surface of the cover 250 of a device. The structure further includes a circuit board 202 parallel to the surface radiator, on that surface of the circuit board wich is visible in FIG. 2 being a strip-like feed conductor 216 of the antenna on the opposite side of the circuit board 202, i.e. on the surface facing the surface radiator, there is a conductive plane 210 with a slot-like nonconductive area 220. The center conductor of the feed line 205 is connected to the conductive strip 216 and the sheath to the conductive plane 210 which is thus connected to the signal ground. The antenna is matched by choosing appropriate dimensions for the circuit board 202 with its conductive parts. Moreover, dimensions of the structure are chosen such that the slot 220 resonates in the operating band and radiates energy to the surface radiator 230. As the surface radiator, in turn, resonates, it radiates radio-frequency energy into its surroundings.
Antennas like the one depicted in FIG. 2 are used in some mobile network base stations, for example. It is conceivable that such an antenna would be applied in mobile stations as well. An advantage of such a structure would be that the antenna could be matched without needing to shape the radiator proper. However, little or no space would be saved compared to the structure shown in FIG. 1. Furthermore, such an antenna structure would have only one operating band, which would be a disadvantage.